Primary (non-rechargeable) electrochemical cells having an anode of lithium are known and are in widespread commercial use. The anode is comprised essentially of lithium metal. Such cells typically have a cathode comprising manganese dioxide, and electrolyte comprising a lithium salt such as lithium trifluoromethane sulfonate (LiCF3SO3) dissolved in anonaqueous solvent. The cells are referenced in the art as primary lithium cells and are generally not intended to be rechargeable. A common primary lithium cell has a lithium anode and cathode comprising MnO2 (Li/MnO2 cell) used to power 35 mm cameras. Alternative primary lithium cells with lithium metal anodes but having different cathodes are also known. Such cells, for example, have cathodes comprising iron disulfide (FeS2) and are designated Li/FeS2 cells. The iron disulfide (FeS2) is also known as pyrite.
The Li/MnO2 cells or Li/FeS2 cells are typically in the form of cylindrical cells, typically an AA or AAA size cell or ⅔A size cell having wound electrodes with separator sheet therebetween. The Li/MnO2 cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO2 alkaline cells and also have higher energy density (watt-hrs per cm3 of cell volume) than that of alkaline cells. The Li/FeS2 cells have a voltage (fresh) of between about 1.2 and 1.8 volts which is about the same as a conventional Zn/MnO2 alkaline cell. However, the energy density (watt-hrs per cm3 of cell volume) of the Li/FeS2 cell is much higher than a comparable size Zn/MnO2 alkaline cell. The theoretical specific capacity of lithium metal is high at 3861.4 mAmp-hr/gram and the theoretical specific capacity of FeS2 is 893.6 mAmp-hr/gram. The FeS2 theoretical capacity is based on a 4 electron transfer from 4Li per FeS2 molecule to result in reaction product of elemental iron Fe and 2Li2S. That is, 2 of the 4 electrons reduce the valence state of Fe+2 in FeS2 to Fe and the remaining 2 electrons reduce the valence of sulfur from −1 in FeS2 to −2 in Li2S. In order to carry out the electrochemical reaction the lithium ions, Li+, produced at the anode must transport through the separator and electrolyte medium and to the cathode.
Overall the Li/FeS2 cell is much more powerful than the same size Zn/MnO2 alkaline cell. That is, for a given continuous current drain, particularly for higher current drain over 200 milliAmp, as reflected by the voltage vs. time discharge profile the voltage drops off much less quickly for the Li/FeS2 cell than the Zn/MnO2 alkaline cell. This results in a higher energy output obtainable from a Li/FeS2 cell compared to that obtainable for a same size alkaline cell. The higher energy output of the Li/FeS2 cell is also clearly shown more directly in graphical plots of energy (Watt-hrs) versus continuous discharge at constant power (Watts) wherein fresh cells are discharged to completion at fixed continuous power outputs ranging from as little as 0.01 Watt to 5 Watt. In such tests the power drain is maintained at a constant continuous power output selected between 0.01 Watt and 5 Watt. (As the cell's voltage drops during discharge the load resistance is gradually decreased raising the current drain to maintain a fixed constant power output.) The graphical plot Energy (Watt-Hrs) versus Power Output (Watt) for the Li/FeS2 cell is considerably above that for the same size alkaline cell. This is despite that the starting voltage of both cells (OCV) is about the same, namely, between about 1.2 and 1.8 volt.
Thus, the Li/FeS2 cell has the advantage over same size alkaline cell, for example, AAAA, AAA, AA, C or D size or any other size in that the Li/FeS2 may be used interchangeably with the conventional Zn/MnO2 alkaline cell and will have greater service life, particularly for higher power demands. Similarly the Li/FeS2 cell which is primary (nonrechargeable) can be used as a replacement for the same size rechargeable nickel metal hydride cell, which has about the same voltage (fresh) as the Li/FeS2 cell.
The electrochemical cell's anode and cathode may be balanced so that the theoretical capacity (mAmp-hr) of either the anode or cathode is in excess. For example, Zn/MnO2 alkaline cells are typically balanced so that the theoretical capacity of the cathode is greater than the theoretical capacity of the anode. See, e.g. U.S. Pat. No. 6,585,881 B2 wherein it is stated that the ratio of theoretical capacity of the cathode to the theoretical capacity of the anode is about 1.05 at col. 15, lines 33-36. In U.S. Pat. Nos. 6,849,360 B2 and 7,157,185 B2 it is indicated that in the Li/FeS2 cell the anode and cathode should be balanced so that the “input ratio” of anode to cathode is less than or equal to 1.0. The term input ratio as used in the U.S. Pat. Nos. 6,849,360 and 7,157,185 references computes to the same value as the theoretical capacity ratio of anode to cathode. Thus, the two terms “input ratio” based on interfacial area and “anode to cathode theoretical capacity ratio” are equivalent, except that in the '360 and '185 patents the theoretical capacity of the cathode is based entirely on FeS2 being the only electrochemically active material therein. There are no other cathode active materials, other than FeS2, contemplated in these latter two references so the theoretical cathode capacity of the cathode is based only on the presence of FeS2 in the cathode.
A better definition of the term theoretical capacity of the anode involves computing the ideal capacity (mAmp-hrs) of all the anode active materials therein, and the theoretical capacity of the cathode involves computing the ideal capacity (mAmp-hrs) of all the cathode active materials therein. It shall be understood that the use of such terms theoretical capacity of anode and theoretical capacity of cathode as used in the present application shall be so defined. The “anode active” materials and “cathode active” materials are defined as the materials in the anode and cathode, respectively, which are capable of useful electrochemical discharge. That is, the “anode active materials” and “cathode active materials” promote current flow between the cell's negative and positive terminals when an external circuit between these terminals is connected and the cell is used in normal manner.
In conventional Zn/MnO2 or Li/FeS2 primary cells it is considered desirable to balance the cells so that the theoretical capacity of the cathode is greater than the theoretical capacity of the anode. One reason for this is that the cathode active material discharges less efficiently than the anode active material. That is, the cathode utilization (the percentage of theoretical cathode capacity which is actually attained during cell discharge) is lower for the cathode active material than the anode active material. As a result, the cell is normally balanced so that the cathode is in theoretical capacity excess so that when the cell is fully discharged there will be little, if any, anode active material left in the cell. However, in a Li/FeS2 cell, if the cell is balanced in this manner with cathode in excess as in U.S. Pat. Nos. 6,849,360 B2 and 7,157,185 B2 there is risk of creating discontinuities in the lithium anode surface as the cell continues to discharge. That is, as cell discharge proceeds, the lithium anode layer becomes thinner and thinner and eventually can lead to “severing” of the electrical contact between anode and anode current collector, which electrically connects the anode material to the negative terminal. This of course can result in delay or permanent disruption of cell performance before the expected cutoff voltage is reached.
The Li/FeS2 cell requires and electrolyte formed of a lithium salt dissolved in organic electrolyte solvent, since the lithium anode is highly reactive with water. One of the difficulties associated with the manufacture of a Li/FeS2 cell is the need to add good binding material to the cathode formulation to bind the Li/FeS2 and carbon particles together in the cathode. The binding material must also be sufficiently adhesive to cause the cathode coating to adhere uniformly and strongly to the substrate to which the cathode coating is applied and yet must resist chemical attack by the electrolyte.
The cathode material may be initially prepared in the form of a slurry mixture, which can be readily coated onto the substrate, preferably a metal substrate by conventional coating methods. The electrolyte added to the cell must be a suitable electrolyte for the Li/FeS2 system allowing the necessary electrochemical reactions to occur efficiently over the range of high power output desired. The electrolyte must exhibit good ionic conductivity and also be sufficiently stable, that is, non reactive, with the undischarged electrode materials (anode and cathode components) and also non reactive with the discharge products. This is because undesirable oxidation/reduction reactions between the electrolyte and electrode materials (either discharged or undischarged) could thereby gradually contaminate the electrolyte and reduce its effectiveness or result in excessive gassing. This in turn can result in a catastrophic cell failure. Thus, the electrolyte used in Li/FeS2 cell in addition to promoting the necessary electrochemical reactions, should also be stable to discharged and undischarged electrode materials. Additionally, the electrolyte should enable good ionic mobility and transport of the lithium ion (Li+) from anode to cathode so that it can engage in the necessary reduction reaction resulting in Li2S product in the cathode.
Primary lithium cells are in use as a power source for digital flash cameras, which require operation at higher pulsed power demands than is supplied by individual alkaline cells. Primary lithium cells are conventionally formed of an electrode composite comprising an anode formed of a sheet of lithium or lithium alloy, a cathode formed of a coating of cathode active material comprising MnO2 or FeS2 on a conductive metal substrate (cathode substrate) and a sheet of electrolyte permeable separator material therebetween. The electrode composite may be spirally wound and inserted into the cell casing, for examples, as shown in U.S. Pat. No. 4,707,421. A cathode coating mixture for the Li/FeS2 cell is described in U.S. Pat. No. 6,849,360. A portion of the anode sheet is typically electrically connected to the cell casing which forms the cell's negative terminal. The cell is closed with an end cap which is insulated from the casing. The cathode sheet can be electrically connected to the end cap which forms the cell's positive terminal. The casing is typically crimped over the peripheral edge of the end cap with insulator disk therebetween to seal the casing's open end. The cell may be fitted internally with a PTC (positive thermal coefficient) device or the like to shut down (increase the internal resistance) of the cell in case the cell is exposed to abusive conditions such as short circuit discharge or overheating.
The anode in a Li/FeS2 cell can be formed by laminating a layer of lithium on a metallic substrate such as copper. However, the anode may be formed of a sheet of lithium or lithium alloy without any substrate.
The electrolyte used in a primary Li/FeS2 cells is formed of a “lithium salt” dissolved in an “organic solvent”. Representative lithium salts which may be used in electrolytes for Li/FeS2 primary cells are referenced in U.S. Pat. Nos. 5,290,414 and 6,849,360 B2 and include such salts as: Lithium trifluoromethanesulfonate, LiCF3SO3 (LiTFS); lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N (LiTFSI); lithium iodide, LiI; lithium bromide, LiBr; lithium tetrafluoroborate, LiBF4; lithium hexafluorophosphate, LiPF6; lithium hexafluoroarsenate, LiAsF6; Li(CF3SO2)3C, and various mixtures. In the art of Li/FeS2 electrochemistry, lithium salts are not simply interchangeable as specific salts are workable with specific electrolyte solvent mixtures.
In U.S. Pat. No. 5,290,414 (Marple) is reported use of a beneficial electrolyte for FeS2 cells, wherein the electrolyte comprises a lithium salt dissolved in a solvent comprising 1,3-dioxolane in admixture with a second solvent which is an acyclic (non cyclic) ether based solvent. The acyclic (non cyclic) ether based solvent as referenced may be dimethoxyethane (DME), ethyl glyme, diglyme and triglyme, with the preferred being 1,2-dimetoxyethane (DME). As given in the example the 1,2-dimethoxyethane (DME) is present in the electrolyte in substantial amount, i.e., at either 40 or 75 vol. % (col. 7, lines 47-54). A specific lithium salt ionizable in such solvent mixture(s), as given in the example, is lithium trifluoromethane sulfonate, LiCF3SO3. Another lithium salt, namely lithium bistrifluoromethylsulfonyl imide, Li(CF3SO2)2N also mentioned at col. 7, line 18-19. The reference teaches that a third solvent may optionally be added selected from 3,5-dimethlyisoxazole (DMI), 3-methyl-2-oxazolidone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), tetrahydrofuran (THF), diethyl carbonate (DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfate (DMS), and sulfolane (claim 19) with the preferred being 3,5-dimethylisoxazole.
In U.S. Pat. No. 6,849,360 B2 (Marple) is disclosed an electrolyte for an Li/FeS2 cell, wherein the electrolyte comprises the salt lithium iodide dissolved in the organic solvent mixture comprising 1,3-dioxolane (DX), 1,2-dimethoxyethane (DME), and small amount of 3,5 dimethylisoxazole (DMI). (col. 6, lines 44-48.)
In US 2007/0202409 A1 (Yamakawa) it is stated with reference to the electrolyte solvent for the Li/FeS2 cell at para. 33: “Examples of the organic solvent include propylene carbonate, ethylene carbonate, 1,2-dimethoxy ethane, γ-butyrolactone, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3 dioxolane, sulfolane, acetonitrile, dimethyl carbonate, and dipropyl carbonate, and any one of them or two or more of them can be used independently, or in a form of a mixed solvent.” Such statement is misleading, since the art teaches only specific combinations of electrolyte solvents will be workable for the Li/FeS2 cell depending on the particular lithium salt to be dissolved in the solvent. (See, e.g. above U.S. Pat. Nos. 5,290,414 and 6,849,360). The reference Yamakawa does not teach which combination of solvents from the above list are to be used with any given lithium salt.
Thus, it should be evident from the above representative references that the choice of a particular organic solvent or mixture of different organic solvents for use in conjunction with any one or more lithium salts to produce a suitable electrolyte for the Li/FeS2 cell is challenging. This is not to say that many combinations of lithium salts and organic solvents do not produce a Li/FeS2 cell will not work at all. But rather the challenge associated with such cells using an electrolyte formed with just any combination of known lithium salt and organic solvent is that the problems encountered will likely be very substantial, thus making the cell impractical for commercial usage. The history of development of lithium cells in general, whether lithium primary cells, e.g. non rechargeable Li/MnO2 or Li/FeS2 cells or rechargeable lithium or lithium ion cells reveals that just any combination of lithium salt and organic solvent cannot be expected to result in a good cell, that is, exhibiting good, reliable performance. Thus, references which merely provide long lists of possible organic solvents for Li/FeS2 cells do not necessarily teach combinations of solvents or combination of specific lithium salts in specific solvent mixtures, which exhibit particular or unexpected benefit.
Accordingly, it is desired to produce a Li/FeS2 cell with a cathode having improved utilization (efficiency) which can enable benefits in improved cell performance.
It is desired to balance the FeS2 cell so that the lithium anode material is in theoretical capacity (mAmp-hrs) excess in relation to the theoretical capacity of the cathode, thereby reducing the risk of severing of the electrical connection between the anode material and anode current collector as the anode lithium layer becomes increasingly thinner during cell discharge.
It is desired to increase the anode/cathode interfacial surface area in a Li/FeS2 wound cell resulting in thinner cathode without sacrificing capacity.
It is desired to produce a primary (nonrechargeable) Li/FeS2 cell having good rate capability so that the cell may be used in place of rechargeable batteries to power digital cameras.